1. Field of the Invention
The present invention generally relates to methods and apparatus for drilling in a substantially horizontal subsurface zone, and more particularly, to methods and apparatus for detecting the approach of stratum bordering the subsurface zone.
2. Related Application
This application incorporated much of the U.S. application Ser. No. 07/715,401, filed on Jun. 14, 1991, entitled WELL LOGGING METHOD AND APPARATUS PROVIDING MULTIPLE DEPTH OF INVESTIGATION USING MULTIPLE TRANSMITTERS AND SINGLE RECEIVER PAIR, in the names of M. Vikram Rao, Roger P. Bartel, and Paul F. Rodney, now U.S. Pat. No. 5,278,507.
3. Description of the Prior Art
In recent years, there has been a substantial increase in the drilling of "horizontal" wells. Such wells often have much greater productivity than the more standard "vertical" wells. It is well known in the art that these "horizontal" wells are not necessarily horizontal but rather have boreholes which follow within the boundaries of a producing subsurface zone which deviates from horizontal to some degree.
In the process of drilling such a borehole, it becomes necessary to guide the drill bit so that the borehole does not leave the boundaries of the subsurface producing zone. A boundary of a producing zone may be established by various non-oil bearing formations or it may be established by such borders as the oil-water contact level in the same producing formation. In order to avoid these boundaries and stay within the producing formation, means have been developed in the prior art, with varying success, to detect and subsequently avoid the various boundary stratum.
Two methods for detecting a boundary stratum are illustrated, respectively, in U.S. Pat. Nos. 4,786 874 and 4,601,353. Each of these methods employs a directionally focused sensor. One method generally describes a directionally focused gamma ray tool and the other method describes a directionally focused resistivity tool. These tools show a change in sensor readings as a boundary stratum is approached. The drill string may then be rotated as necessary to determine the position of the boundary stratum by the variation in magnitude of the sensor readings. Once the position of the boundary stratum is known, the driller can orient the bit to drill away from the boundary stratum.
In a paper entitled "Response of 2 MHz LWD Resistivity and Wireline Induction Tools in Deipping Beds and Laminated Formations", presented by Barbara Anderson et al at the SPWLA 31st Annual Logging Symposium in Jun. 24-27, 1990, there is a general review of comparing the responses of wireline induction and 2 MHz MWD tools in dipping formations.
While these methods have added to the art, they do have shortcomings. One problem with the above methods is that they operate under the assumption that a particular zone, including the borehole, is relatively homogenous as to its resistivity or its natural gamma radiation, which may or may not be true.
For instance, if the borehole, for some reason such as the particular type of formation or drilling fluid, begins to washout so that its size increases, the prior art tools would most likely signal falsely to the driller that the path of the borehole needs to be altered due to the approach of a boundary stratum. The prior art does not provide a way to distinguish such an occurrence from the actual approach of a boundary stratum. Unnecessary turning in the borehole can create future problems with the drilling or perhaps with running casing through the borehole, as well as tending to slow the speed of drilling.
Another common borehole phenomenon which can cause a false signal in the prior art tools is an accumulation of mud solids which sometimes builds up on the drilling bit, particularly in horizontal wells. These accumulations, called mudballs in the trade, have a tendency to scrape off on the face of porous and permeable rock which typically form a producing formation.
Also, if a change in a characteristic of the formation as a whole occurred, such as a generally decreasing resistivity, the prior art would have difficulty, or at least require considerable time, to distinguish this occurrence from the approach of a boundary stratum.
In some cases, while drilling through horizontal producing zones, the drillers main concern may be with the oil-water contact boundary stratum rather than other boundary stratum on the sides or from above the producing zone. The driller may wish to keep the borehole a certain distance above the oil-water contact level so as to maximize the productive life of the well. Also, the driller will probably not want to turn upwards unnecessarily. In such a case, the driller does not necessarily need a directionally focused sensor to tell him in which direction the boundary stratum is located because he already has reasonable certainty that the boundary stratum lays below the present borehole path. In fact, if the motor type drilling assembly is being used, due to the occasional necessity to change the direction of the bit, a tool with a directionally focused sensor may be focused in the wrong direction to indicate the approach of an oil-water contact boundary stratum and therefore be unreliable. Moreover, the need to reorient the tool may create undesirable drilling operations. On the other hand, the driller does need a sensor tool that will be less subject to giving false signals than the prior art tools.
In the above situation, the driller may wish to use a fixed directionally focused sensor for some purpose such as a greater depth of investigation being available. However, even then the driller requires a means for preventing spurious signals which so frequently occur with the prior art tools.
The prior art provides no acceptable method for calculating the approximate angle or dip of an approaching boundary stratum. Such information would generally be useful to the driller for various reasons. It might affect the degree of turn the driller wishes to achieve. The driller will generally desire to make the borehole as straight as possible and avoid making relatively sharp turns for such reasons as given above. Normally, the driller will want to make no more of a turn than is necessary to avoid the boundary statum.
Since information concerning dips or angles of the boundary stratum with the borehole is not available, there presently exists no system which is capable of portraying on a surface terminal or printout a schematic of the borehole along with any formations detected while drilling.
Other problems arise from relying on gamma sensors and resistivity sensors. For instance, a gamma ray tool cannot normally distinguish the oil-water contact level in the same producing zone. As previously stated, this is a boundary stratum the driller desires to avoid. Moreover, a gamma ray device generally has a depth of investigation of only a few inches, thereby providing essentially no useful information for this purpose.
Generally, in a vertical well, a resistivity tool can sense the oil-water contact level but usually requires the water to be somewhat saline so as to produce a clear contrast between the resistivities of the oil zone and the water zone. If the connate water, or water entrapped in the interstices of the formation rock, is fresh, or low in dissolved salts, it may be difficult or even impossible for a resistivity tool to distinguish the oil-water contact. In such a case, the oil-water contact level is oftentimes distinguishable with high frequency electromagnetic sensors that measure dielectric constants, oil and water having different dielectric constants.
Furthermore, U.S. Pat. No. 4,601,353, using the gamma ray tool, is probably not suitable to drilling a well with a rotary drilling assembly as compared to drilling with a motor type drilling assembly. The gamma ray signal, being naturally subject to statistical variations, has a tendency to average out during rotations making neighboring beds difficult to see.
Consequently, a need exists for improvements in techniques for detecting the approach of boundary stratum while drilling horizontal wells which will result in greater reliability and dependability of operation.